1. Field of the Invention
The present invention relates to an image forming method in which in an image forming apparatus such as a copying machine or a printer, image formation is carried out using an amorphous silicon photoreceptor having an amorphous silicon photosensitive layer composed of amorphous silicon, which is characterized in that charging properties in the amorphous silicon photoreceptor are improved in carrying out image formation through the charging, exposing, developing, transferring and charge eliminating steps to stably form a good image at high speed.
2. Description of the Related Art
Conventionally in forming an image in an image forming apparatus such as a copying machine or a printer, an amorphous silicon photoreceptor using amorphous silicon has been used in addition to an organic photoreceptor using selenium or the like as a material composing its photosensitive layer and an organic photoreceptor using an organic material as the material.
When an image is formed using such an amorphous silicon photoreceptor, the surface of the amorphous silicon photoreceptor is charged by a charger, the surface of the photoreceptor thus charged is then subjected to exposure corresponding to image information to form an electrostatic latent image, toner is supplied to the electrostatic latent image thus formed from a developing device to form a toner image on the surface of the photoreceptor, the toner image is transferred onto a recording medium by a transferer or the like to form the toner image on the recording medium, while the toner remaining on the surface of the photoreceptor after the transfer is removed by a cleaning device, and light is then irradiated onto the surface of the photoreceptor from a charge eliminating device to eliminate charge remaining on the surface of the photoreceptor, as in the case of the other photoreceptor.
The above-mentioned amorphous silicon photoreceptor can be utilized at higher speed and over a longer period, as compared with the other photoreceptor, because its photosensitive layer has a high hardness, and has high sensitivity and high charge transporting properties.
In the case of the amorphous silicon photoreceptor, however, the photosensitive layer has a lot of dangling bonds. Part of carriers produced by light are acquired by the dangling bonds, so that the movability of the carriers is decreased, and the probability that the carriers are re-combined is decreased. Consequently, the carriers are liable to remain as an optical memory. When the amorphous silicon photoreceptor is repeatedly used, therefore, the carriers produced in the previous exposing step remain as an optical memory until the surface of the photoreceptor is then charged, so that image noise referred to as "ghost" occurs.
Conventionally in the charge eliminating step for eliminating charge remaining on the surface of the amorphous silicon photoreceptor, therefore, light having a depth in entrance which is approximately the same as the depth in entrance of light having a main wavelength for performing exposure is irradiated as light irradiated from the charge eliminating device to eliminate the above-mentioned charge.
When the light having a depth in entrance which is approximately the same as the depth in entrance of light having a main wavelength for performing exposure is thus irradiated, however, a lot of latent carriers are produced inside the photosensitive layer.
When an image is formed at high speed, therefore, the subsequent charging is performed by the charger before the carriers thus produced are re-combined. The carriers are moved at the time of the charging, so that a surface potential on the photoreceptor is decreased, resulting in significantly decreased charging properties in the photoreceptor.
If charging conditions by the charger are made severe in order to sufficiently charge the amorphous silicon photoreceptor, a part of the photosensitive layer is subjected to dielectric breakdown, producing a pin hole. Consequently, noise occurs in the formed image.
In order to improve charging properties in such an amorphous silicon photoreceptor, it has been proposed that the thickness of its photosensitive layer is increased, and a time period from charge elimination using light to charging is set to not less than 200 msec, to recombine latent carriers produced inside its photosensitive layer by the charge elimination using light in a period elapsed until the subsequent charging is performed, as disclosed in Japanese Patent Laid-Open No. 41155/1986.
Even when the thickness of the photosensitive layer is increased as described above, however, the charging properties in the amorphous silicon photoreceptor cannot be sufficiently improved. Further, in the above-mentioned gazette, the time period from charge elimination using light to charging is only specified, and a time period from charging to development is not described. Moreover, if the rotational speed of the amorphous silicon photoreceptor is increased in order to form an image at high speed, it is significantly difficult to ensure not less than 200 msec as the time period from charge elimination using light to charging from the relationship with the other steps such as the developing and transferring steps in an electrophotographic process.